Wild house mice (Mus musculus) reduce their water budget to 40% of controls through gradual acclimation to water restriction. It is probable that a transition of this magnitude involves alteration of body fluid composition and volume. The objectives are to find whether: 1. such alteration does occur; 2. the alteration is adaptive or pathological. Basic techniques will be isotope dilution for volumes and chemical /physical methods for ions, protein, and osmolality. The study will provide further comprehension of the mechanisms of acclimation and will allow precise formulation of future experiments on: 1. tissue adaptation and osmoreceptor accommodation; 2. hormonal actions in normally hydrated and dehydrated animals; 3. mechanisms of reduced evaporation.